This invention relates to an integrated filter arrangement comprising a resistor section coupled between the input and the output of the arrangement and a capacitor section connected across the output of the arrangement.
Such filter arrangements are generally known per se. If such filters are to be integrated, problems are bound to arise as the resistance and capacitance values increase, in particular because components having comparatively large values also occupy a comparatively large substrate area.
A solution to this problem is the use of transconductor filters as is described, for example, by J.O. Voorman et al in the article "Bipolar Integration of Analog Gyrator and Laguerre Type Filters (Transconductor-Capacitor Filters)", Proceedings of the sixth European Conference of Circuit Theory and Design, Sept. 6-8, 1983, VDE-Verlag GmbH, Berlin Offenbach. In a transconductor filter the resistance is simulated by means of a kind of differential amplifier. The value of the resistor thus simulated is equal to the inverse of the transconductance of the differential amplifier. The resistance value is adjusted by varying the tail current of the differential amplifier and in principle it can be defined accurately.
For several uses and in particular for d.c. control loops, low-pass filters are needed to provide a maximal rejection of a.c. components superimposed on the direct voltage. Such a filter should provide a maximal rejection of spurious signals rather than an accurately defined cut-off frequency. In practice this means that the cut-off frequency should be below a specific value. For many uses it is required, for example, that the cut-off frequency be lower than 10 Hz. If such a filter is to be integrated completely the customary integration techniques will impose a limit of approximately 1 nF on theum maximum capacitance value. This means that in order to obtain a cut-off frequency lower than 10 Hz a resistance value of at least 16 m.OMEGA. is needed. In the case of a transconductor filter the tail current of the differential amplifier used therein is required to have value of 6 nA or less. This is such a low current level that leakage currents in the integrated circuit are likely to impair the correct operation of the filter. Moreover, the 1 nF capacitor occupies a comparatively large surface area on the IC (typically approximately 1 mm.sup.2). Realising such filters by means of a filter circuit of the transconductor type therefore poses several problems.